In many parts of the world, including Canada and the United States, fuel safety Standards, Codes, Acts, etc. require that a burner with a flame space greater than a certain distance be sensed at the farthest point of the ignition. For example, in some jurisdictions, “when a burner has a flame width (or runs the length of a burner) in excess of 3 ft (900 mm) from the source of the burner ignition, (a) the main burner flame shall be proven at the farthest point(s) along its base from the source of ignition; (b) the source of ignition shall be located in the combustion zone adjacent to the entry of the fuel or fuel/air mixture to the burner; and (c) the main burner flame shall be proven at a location providing the most stable flame detection at all firing rates and not affected by the source of ignition.” It is also required that line, pipe, ribbon, and radiant burners that are installed adjacent to one another or connected with flame-propagating devices, shall be considered to be a single burner and shall have at least one flame detector installed to sense burner flame at the end of the assembly farthest from the source of ignition.
One well known technique for detecting the presence of a flame is by using a flame rod, which works based on electrical properties associated with the flame. As a flame burns, it produces an ionized region in its vicinity, thereby providing an electrically conductive medium. This property can be utilized in conjunction with a probe placed into the flame, and a grounded metal burner to produce a usable electrical signal. If such apparatus is constructed with an effective grounded burner area greater than the effective probe area, typically in at least a 4 to 1 ratio, the flame will exhibit electrical characteristics somewhat similar to those of a diode in series with a 10 Mega-ohm resistor. If an alternating current signal is injected into the flame by the probe, the signal will be rectified by the flame. Appropriate filtering and amplification circuitry may then be employed to extract the rectified signal.
In the line, pipe, ribbon and radiant heaters, special flame rods are fabricated and fastened to the body of the burner. These rods run the length of the burner body, and at the end of the flame space the rods protrude into the flame. Flame rods can be fabricated into various lengths to suit the application.
Typically flame rods are made using a conductor, such as a high temperature alloy like Kanthal, Stainless Steel, and Inconel rod. The rod is usually surrounded by an insulator (ceramic, steatite etc.) to protect the rod from grounding. The principles of flame rectification require that flame sensing rod is to tell the controller that the main gas burners have ignited and a flame is present. If no flame is present after a certain amount of time, the controller needs to close the gas valves to the burners.
Typically, a burner or multiple burners are installed in a chamber and there is no direct access to the opposite end of the burner assembly. Thus, sensing the flame at the farthest point from ignition becomes very difficult. If a flame rod is mounted at the end of a burner, service and maintenance become very difficult.
The problem is that most currently available devices are not reliable for numerous reasons. They use a stainless steel, Kanthol or Inconel rods that run the length of the burner. Along the way the rod has several ceramic insulators that are fastened to brackets that allow the rod to remain fixed. At the end of the rod a portion is positioned into the flame. These ceramic insulators often move or crack, and as a result, the flame rod grounds and can no longer send an electrical signal to the burner flame safeguard. In addition, depending on who makes the assembly, some providers add extra steel over the area of the flame. This is in hopes that by providing a greater surface area over the flame it will sense better. Often, during operation as the steels expands when heated, it can touch the burner surface, causing it to ground out and not sense the flame. In this case, it will not re-ignite as the flame relay will not hold open, as when it is grounded the flame cannot sense during the trial for ignition period. In order to fix a failed flame rod, the burner needs to be removed from the chamber and a new assembly be installed, in the hopes that when reinstalled it will work. If not, the burner needs to be removed and process repeated.
Most manufacturers of such components are always pushing the envelope with the design of flame rods and only try to reduce the amount of ceramic (insulators) used. In addition, developers look for more robust materials or alloys that can handle exposure to high temperatures.